1. Field of the Invention
This invention relates to a data transferring method for transferring data detected by a plurality of sensors to a detected data receiver. In particular, this method is applied where a plurality of sensors are connected to a bus connection type serial communication network, and achieves efficient transfer of data detected by these sensors to a detected data receiver.
2. Description of the Prior Art
FIG. 6 is a diagram showing the structure of a device for controlling the position of a movable table. This structure is commonly applied to the devices employing a conventional data transferring method (described later) and the data transferring method of the present invention.
As shown in the drawing, a plurality of sensors are used in this structure so as to ensure accurate control of the position. For instance, a motor 5 is provided with a rotary encoder 1, which is a sensor for detecting the rotation position and speed of the rotor shaft 6 of the motor 5. The rotor shaft 6 is directly connected to a ball screw 7, so that the table is moved linearly due to the rotation of the ball screw 7. The linear movement of the table 8 is detected by another sensor, i.e., a linear encoder 2. For more accurate positional control, a temperature sensor 3 is provided. According to the temperature measured by this sensor 3, the expansion rate of the ball screw 7 is taken into consideration in the control. These sensors are all connected to a bus connection type serial communication network 9, so that detected data are transferred to a servo motor controller 4 via the serial communication network 9. The servo motor controller 4 controls the position of the table 8 based on the data detected by the above sensors. Conventionally, detected data transferring has been controlled by means of exchanging communication data as shown in the timing charts of FIG. 7 or 8, in which the time axis extends to the right.
In the method relating to the timing chart of FIG. 7, the servo motor controller 4 sequentially transmits detected data transfer request commands R1, R2, and R3 to the respective sensors for polling. That is, the servo motor controller 4 waits for the completion of detected data transferring by any one of the sensors before it transmits another transfer request command to the next sensor. With this arrangement, signal collision in the network is prevented. In this method of transmission, transfer request commands R1, R2, and R3 respectively addressing the rotor encoder 1, a linear encoder 2, and the temperature sensor 3 and detected data SD1, SD2, and SD3 respectively outputted by those sensors are sequentially and alternately transmitted via the serial communication network 9.
In the method relating to the timing chart shown in FIG. 8, all of the sensors are set beforehand with data on a required elapsed time from the receipt of a command RQ to the start of transferring detected data. In this event, elapsed times T1, T2, and T3 for respective sensors are determined different from one another and, moreover, they need to be somewhat long so as to secure sufficient time to prevent collision of transferring data in the network. In this method of transmission, when the servo motor controller 4 outputs one transfer request command RQ to the serial communication network 9, all of the sensors involved, i.e., the rotary encoder 1, the linear encoder 2, and the temperature sensor 3, detect this command RQ, and output, in this order, detected data SD1, SD2, and SD3 in time T1, T2, and T3, respectively.
The transferring method relating to FIG. 7 is advantageous in that the transfer procedure of detected data can be flexibly changed since transfer request commands are transmitted individually to the respective sensors. However, frequent transmission of such commands results in the problem of imposing a heavy load on a detected data receiver, or the servo motor controller 4. More seriously, a high occupancy rate of transfer request commands in a network causes a drop in the efficiency of transferring detected data.
The transferring method relating to FIG. 8, on the other hand, lacks flexibility because detected data must be transmitted from the respective sensors only in a predetermined order, although it can result in a reduced rate of transmission of transfer request commands. Still more seriously, since a preset time for each sensor is defined as a rather long time in order to prevent data collision in the network, transmission efficiency cannot be improved as much as would be expected, in spite of reduced number of command transmissions.